The present invention relates in general to centrifugal pumping devices for circulatory assist and other uses, and, more specifically, to an improved startup of a magnetically-levitated impeller that avoids excessive wear of the impeller against the housing before levitation is obtained.
Many types of circulatory assist devices are available for either short term or long term support for patients having cardiovascular disease. For example, a heart pump system known as a left ventricular assist device (LVAD) can provide long term patient support with an implantable pump associated with an externally-worn pump control unit and batteries. The LVAD improves circulation throughout the body by assisting the left side of the heart in pumping blood. One such system is the DuraHeart® LVAS system made by Terumo Heart, Inc., of Ann Arbor, Mich. The DuraHeart® system employs a centrifugal pump with a magnetically levitated impeller to pump blood from the left ventricle to the aorta. The impeller acts as a rotor of an electric motor in which a rotating magnetic field from a multiphase stator couples with the impeller and is rotated at a speed appropriate to obtain the desired blood flow through the pump.
A control system for varying pump speed to achieve a target blood flow based on physiologic conditions is shown in U.S. Pat. No. 7,160,243, issued Jan. 9, 2007, which is incorporated herein by reference in its entirety. The stator of the pump motor can be driven by a pulse-width modulated signal determined using a field-oriented control (FOC) as disclosed in U.S. application Ser. No. 13/748,780, filed Jan. 24, 2013, entitled “Impeller Position Compensation Using Field Oriented Control,” which is incorporated herein by reference in its entirety.
The centrifugal pump employs a sealed pumping chamber. By levitating the impeller within the chamber when it rotates, turbulence in the blood is minimized. The spacing between the impeller and chamber walls minimizes pump-induced hemolysis and thrombus formation. The levitation is obtained by the combination of a magnetic bearing and a hydrodynamic bearing. For the magnetic bearing, the impeller typically employs upper and lower plates having permanent magnetic materials for interacting with a magnetic field applied via the chamber walls. For example, a stationary magnetic field may be applied from the upper side of the pump housing to attract the upper plate while a rotating magnetic field from the lower side of the pump housing (to drive the impeller rotation) attracts the lower plate. The hydrodynamic bearing results from the action of the fluid between the impeller and the chamber walls while pumping occurs. Grooves may be placed in the chamber walls to enhance the hydrodynamic bearing (as shown in U.S. Pat. No. 7,470,246, issued Dec. 30, 2008, titled “Centrifugal Blood Pump Apparatus,” which is incorporated herein by reference). The magnetic and hydrodynamic forces cooperate so that the impeller rotates at a levitated position within the pumping chamber. Since the hydrodynamic forces change according to the rotation speed of the impeller, the magnetic field may be actively controlled in order to ensure that the impeller maintains a centered position with the pumping chamber.
Prior to starting rotation of the impeller, the axial forces acting on it are not balanced. Magnetic attraction causes the impeller to rest against one of the upper or lower chamber walls. In many pump designs, it is possible for the impeller to be arbitrarily resting against either one of the walls. When rotation begins, the rubbing of the impeller against the chamber wall can cause undesirable mechanical wear of the impeller and/or wall. The amount of wear is proportional to the rotation angle traversed until the impeller lifts off of the pump housing and to the normal force between the impeller and housing.
In a typical startup sequence of the prior art, the stator coils are energized to produce a strong, stationary magnetic field that rotates the impeller into alignment with a known phase angle. When the impeller moves during alignment, it typically overshoots the desired position due to the strong field and then it oscillates around the desired position until the motion dampens out. Much mechanical wear can occur during this step. Once in the aligned position, the field-oriented control can begin closed-loop control to accelerate the impeller until the bearing forces separate it from the chamber wall. However, the normal force can be high before separation occurs, further increasing the wear.